fix: browser env mutation.handlers is undefined (#892)

* Fix browser env mutation.handlers is undefined

In the latest Firefox and Google browsers, state data tree will not load for error:[mutation.handlers is undefined], because mutation handler actually passes is null (store. _mutations is empty) while pushing the handlers value as an element to the base mutations array, just add robust check.

* Fix CI lint space problem

src/backend/vuex.js

@@ -146,7 +146,10 @@ export function initVuexBackend (hook, bridge) {
       // Replay mutations
       for (let i = snapshot.index + 1; i <= index; i++) {
         const mutation = mutations[i]
-        mutation.handlers.forEach(handler => handler(mutation.payload))
+        if (mutation.handlers) {
+          mutation.handlers.forEach(handler => handler(mutation.payload))
+        }
+
         if (i !== index && i % SharedData.cacheVuexSnapshotsEvery === 0) {
           takeSnapshot(i, state)
         }